The development and management of effective thermal solutions becomes increasingly important as the performance thresholds of CPU's (central processing units) rise. The objective of thermal management is to ensure that the temperature of each component in a system is maintained within specified functional limits. The functional temperature limit is the range within which electrical circuits can be expected to meet their specified performance requirements. Operation outside the functional limit can degrade system performance and cause reliability problems.
Furthermore, case temperatures within a specified range should also be maintained. The case temperature is the surface temperature of the package at its hottest point, typically at the geographical center of the chip. Temperatures exceeding the case temperature limit over a length of time can cause physical destruction or may result in irreversible changes in operating characteristics.
From an acoustics standpoint, there is a strong desire to migrate the PC (personal computer) into quiet home environments, particularly important in areas that have tight living conditions, such as in China & Japan. As manufacturing processes improve to decrease die area and increase heat fluxes, there is a corresponding need for higher performance air-cooling heat sinks. While the solution trend is to increase the efficiency of the fan, RPM (rotations per minute) of the fan, heat sink surface area, and/or volume airflow through the heat sink, these trends all result in increased acoustic noise from the fan.
To address the thermal issues, combinations of heat sinks and fans may be used to cool the CPU and the system. Today's fans that are used to cool CPU's and systems can only slow down to approximately ⅓ of their nominal full speed through the use of pulse width modulation fan speed control. Some fans can only be slowed down to ½ speed. Unfortunately, the nominal noise at full speed is so high that even when the speed is reduced to ½ or ⅓ full speed, the resulting acoustic noise is still too high for quiet environments. A current thermal solution uses a single fan that impinges high-speed air directly on the CPU heat sink. However, this concentrated cooling results in high acoustic noise.
High performance processors dissipate such large amounts of power that they must additionally dissipate heat through the use of extended surfaces such as heat sinks to maintain their reliability. However, even the use of heat sinks does not alleviate the need for the use of fans, and therefore does not contribute to resolving the acoustic problem associated with the use of fans.